In the past, the computer industry has relied extensively on magnetic media in many forms, such as 8 or 5 1/4 inch diskettes or hard disk platters, for data and information storage. This magnetic media storage involves using specialized recording elements or heads positioned adjacent to the media to create magnetic fields that penetrate the media surface and alter the orientation of magnetic fields resident in the media. Using this technique, data is recorded as the leading edge of a series of north/south or south/north transitions in the media.
Magnetic media are generally organized into a series of adjacent "tracks" which are further subdivided into predetermined patterns such as sectors and clusters. Predefined formats of a certain number of tracks per inch on a medium and a certain number of magnetic transitions per track allows a storage system to maintain synchronous control and reliability in data transfer to and from the medium. To increase the linear recording density of data storage within a track, the number of transitions per track can be increased. This has been successful especially with newly developed media which have higher magnetic retention values and increased magnetic field resistance laterally across the tracks. Within a given track, timing techniques make it possible to obtain transition spacings on the order of 1 micron or less. However, magnetic media have larger track-to-track dimensional and density limitations. This is due to the inability to position read/write apparatus used to exchange data between a magnetic medium and an associated data system relative to inter-track dimensions of several microns or less.
As an alternative, optical means of data storage are being developed. Optical data storage is appealing because optical systems can operate at higher media addressing speeds, greater bandwidths, and smaller track-to-track dimensions as compared to magnetic media. These features are possible because finely focused light is used and can be optically manipulated to smaller dimensions than magnetic read/write apparatus. Small diffraction limited lasers can be positioned laterally across data storage tracks with 1 to 2 micron dimensions. However, optical systems impose new constraints on linear recording density.
For any optically based read/write system the minimum spot size of the light or optical beam on the media must be accounted for. This spot size turns out to be significantly larger than for a magnetic transition and reduces linear recording density. A typical diffraction-limited laser provides a focal or waist size on the order of 1 to 2 microns. An optical system must also provide sufficient (additional) adjacent storage media to resolve between adjacent write patterns left by a laser. At the same time, the minimum pattern size is enhanced by the recording techniques used for optical data storage, which differ from magnetic media.
In optical data storage the data is recorded in terms of features or surface effects created by a laser beam spot (the "write beam") on the media. These surface effects create generally circular patterns that can have many topologies including "bubbles", bumps, pits, and holes. Presently, optical data storage can be divided into two basic classes, the first being material removal and the second being state alteration. For surface removal, the laser energy is used to actually remove material from the media so as to effect a different surface reflectivity or transmissivity. Materials such as thin metallic films on various substrates fall into this class. The state alteration technique uses the laser energy to alter a state or property of the optical media. This includes phase transitions such as amorphous/crystalline transitions, electro-optic effects, or chemical state changes. The more preferred method of state alteration involves an electro-optical state change. By altering the electro-optical properties of a media, the polarization of an incident laser "read beam" is shifted or rotated and the change of polarization detected accordingly.
The regions of altered state can be thought of as circular marks on the optical media. Decreasing mark separation to increase the number of marks per track would seem to obtain higher linear recording density for optically stored data. However, certain inherent limitations are reached very quickly. This can be illustrated using the optical data recording patterns shown in FIG. 1.
For a single mark, high resolution output is obtained whenever the laser beam used for reading marks (the "read beam") aligns itself precisely with a mark. For two adjacent marks high resolution output is obtained when the read beam is well aligned with each mark in sequence. In FIG. 1 a series of adjacent optical marks 10, 12, and 14 are illustrated positioned on an optical media 20 and separated by "non-mark" regions 16 and 18 respectively. The nominal size for a mark is assumed to be 1 micron, the size of a typical write beam. In the case of the first two marks, 10 and 12, mark separation is slightly greater than 1 micron and a read beam will be totally reflected or diffracted by the non-mark region as shown by the dotted outline 16. However, the two marks 12 and 14 are positioned less than micron apart which results in overlap into the mark regions for a read beam reading the non-mark region as shown by dotted line 18. In this case, the detection system begins to lose resolution between the mark and non-mark regions and can erroneously conclude that it is in a mark region. Also the read system begins to lose resolution between the adjacent marks 12 and 14. This is analogous to loss of intersymbol resolution in magnetic media.
Another factor affecting linear recording density in optical media is the selection of a data coding scheme. Pulse-width modulation (PWM) and run-length-limited (RLL) coding are the most commonly employed techniques, having been developed for magnetic data storage. When applied to optical media, the PWM coding employs leading and trailing edge detection of a mark, along with changes in mark sizes, to record data and decrease the number of discrete marks. An RLL encoding scheme encodes data into unique strings of data bits to increase channel data density and decrease data error propagation.
While both of these techniques serve certain advantageous functions, they limit the linear recording density in optical storage media by employing, on average, equal mark and non-mark minimum dimensions. This leaves a fairly large surface area of the optical media unused and creates a natural limit to the linear recording density.
Therefore, what is needed to improve the linear recording density for data recorded on optical storage media is a new method of decreasing the average non-mark area without substantial loss of intersymbol resolution. It would also be an advance in the art to be able to provide improved linear recording density with limited data error propagation.